1. Field of the Invention
The present invention relates to a control method for reducing a pointing error of an antenna having AZ (azimuth angle) axis and EL (elevation angle) axis gimbals and to a control apparatus that uses such a control method.
2. Background Information
In order for a mobile station, such as an airplane or a ship, to conduct communications, it is generally necessary for an antenna to be controlled so that it tracks a communication satellite even when the hull is rocking. Methods for tracking a communication satellite include methods that execute control using an antenna tracking mechanism having two axes, three axes, or four axes.
For example, a two-axis mechanism has a turning axis (hereinafter called AZ axis) that is a vertical axis about which the antenna can be turned through a full circle, and a rotation axis (hereinafter called EL axis) that is a horizontal axis about which the antenna can be tilted up and down. The two-axis mechanism is provided so that the antenna can be rotated about both axes independently.
While such a two-axis mechanism enables a simple antenna mechanism to be achieved, it is generally not capable of tracking when complex rocking occurs. In particular, when the satellite is on the AZ axis, the pointing error becomes extremely large. This pointing error is called a gimbal lock phenomenon and various research efforts are underway regarding this topic.
Japanese Laid-open Patent Publication No. H05-25972, for example, discloses an antenna pointing apparatus that can avoid the gimbal lock phenomenon and point an antenna at a satellite in a favorable manner even when the elevation angle of the satellite is near 90 degrees. The antenna pointing apparatus includes an antenna having a center axis X-X, a support mechanism that supports the antenna so that the antenna can be rotated about an elevation axis, an elevation gyroscope, and an azimuth gyroscope. The antenna pointing apparatus further includes a first accelerometer that detects a tilt angle of the center axis X-X with respect to a horizontal plane, a second accelerometer that detects a slant angle of the elevation axis with respect to the horizontal plane, an azimuth transmitter that detects a rotational angle of an azimuth gimbal about an azimuth axis, and an elevation transmitter that detects a rotational angle of the antenna about the elevation axis with respect to the azimuth gimbal. When the elevation angle of a satellite is in a region near 90 degrees, the apparatus controls the antenna such that the elevation axis is adjusted according to the direction of a tilt axis of a ship hull.
Japanese Laid-open Patent Publication No. H07-154127 discloses an antenna pointing apparatus that can point an antenna at a satellite in a favorable manner at all times, even when a ship hull is rocking or is statically tilted. The antenna pointing apparatus is provided with a tilt compensation computing section that computes a tilt compensation value, a rocking angle computing section that computes rocking angles of a mounting surface of the antenna pointing apparatus, and a gimballing error computing section that computes a gimballing error occurring in a bow directional angle based on a satellite bearing angle and the rocking angles supplied from the rocking angle computing section. An output signal from the gimballing error computing section is used to eliminate the gimballing error from the bow directional angle.
Japanese Laid-open Patent Publication No. H07-154128 discloses an antenna pointing apparatus that can point an antenna at a satellite in a favorable manner at all times, even when a rotational angle θ of the antenna about an elevation axis is close to 90 degrees. The antenna pointing apparatus includes a rocking angle computing section that computes rocking angles of a hull surface, a servo deviation compensation computing section that computes a detected angular velocity deviation between an elevation gyroscope and an azimuth gyroscope, an azimuth servo deviation limiter value computing section that computes an azimuth servo deviation limiter value, and a servo deviation limiter that limits an azimuth servo deviation. The apparatus is configured to prevent an azimuth servo motor from undergoing runaway even when a rotational angle of the antenna about the elevation axis is near 90 degrees.
Among conventional antenna pointing apparatuses having two-axis, three-axis, and four-axis mechanisms, those having three-axis mechanisms and those having four-axis mechanisms are more complex structurally, have a higher parts cost, and cannot be made smaller in size. Consequently, as exemplified in the documents cited above, various two-axis mechanisms have been developed with the objective of reducing size and weight.
However, with the antenna pointing apparatus disclosed in Japanese Laid-open Patent Publication No. H05-25972, the EL axis is “always” fixed in a longitudinal direction of the hull for the satellite near a zenith position. Consequently, it is not possible to satisfy the rocking standard generally required for a satellite communication ship earth station (e.g., an antenna pointing error of ±5 degrees at a pitch angle of 10 degrees and a roll angle of 30 degrees). In other words, the fixing causes the antenna pointing error to be ±10 degrees such that the pointing error does not satisfy the standard when severe rocking occurs.
With the antenna pointing apparatus disclosed in Japanese Laid-open Patent Publication No. H07-154127, it is difficult to accurately recognize rocking of the hull because the elevation gyroscope and the azimuth gyroscope are attached to the antenna. Furthermore, with the antenna pointing apparatus disclosed in Japanese Laid-open Patent Publication No. H07-154128, the computations take a large amount of time, and a large cost of processing is required because a directional angle (or an estimated directional angle) of the satellite is calculated based on an estimated rocking of the hull, and an output of the apparatus is determined based on an amount of difference between the calculated value and the rotation angle about the AZ axis.